Geologia e petrografia das rochas ornamentais de nefelina sienito no Pico do Marapicu, complexo intrusivo de Mendanha, Estado do Rio de Janeiro, Brasil

This paper presents geology and petrography of the nepheline syenite for ornamental use at Marapicu Peak, Mendanha intrusive complex, State of Rio de Janeiro, Brazil, known commercially Granito Cinza As de Paus. There are three types of nepheline syenite products: Classic Type, Modern Type, and Prismatic Amphibole Type. The Classic Type is characterised by soft visual aspect of undulated grey tone. The texture is equigranular and homogenous without mineral orientation. The Modern Type has similar aspects, but has no undulant the grey tone. The Prismatic Amphibole Type is featured by acicular amphibole crystals with notable orientation. The main constituent minerals of these rocks are anorthoclase alkaline feldspar, nepheline, amphibole, and biotite. In spite of the vulnerability to chemical weathering, the rocks are quite fresh because of the effects of weathering passivity. The nepheline syenite has high mechanical strength, with uniaxial compression for rupture superior to common granites for ornamental use. The rock body is massive without notable systems of fractures and faults of tectonic origin, which helps large block extraction. The quantitative colour specification reveals that the soft grey tone is due mainly to the abundance of amphibole, with brightness ca. 15, relative to biotite, ca. 0. The undulated grey tone of Classic Type is originated form metasomatism.